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Living Inside Termites-An Overview of Symbiotic Interactions, with Emphasis on Flagellate Protists

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Living Inside Termites-An Overview of Symbiotic Interactions, with Emphasis on Flagellate Protists Arquipelago - Life and Marine Sciences ISSN: 0873-4704 Living inside termites: an overview of symbiotic interactions, with emphasis on flagellate protists SÓNIA DUARTE, L. NUNES, P.A.V. BORGES, C.G. FOSSDAL & T. NOBRE Duarte, S., L. Nunes, P.A.V. Borges, C.G. Fossdal & T. Nobre 2017. Living inside termites: an overview of symbiotic interactions, with emphasis on flagellate protists. Arquipelago. Life and Marine Sciences 34: 21-43. To degrade lignocellulose efficiently, lower termites rely on their digestive tract’s specific features (i.e., physiological properties and enzymes) and on the network of symbiotic fauna harboured in their hindgut. This complex ecosystem, has different levels of symbiosis, and is a result of diverse co-evolutionary events and the singular social behaviour of termites. The partnership between termites and flagellate protists, together with prokaryotes, has been very successful because of their co-adaptative ability and efficacy in resolving the needs of the involved organisms: this tripartite symbiosis may have reached a physiologically stable, though dynamic, evolutionary equilibrium. The diversity of flagellate protists fauna associated with lower termites could be explained by a division of labour to accomplish the intricate process of lignocellulose digestion, and the ability to disrupt this function has potential use for termite control. Multi-level symbiosis strategy processes, or the cellulolytic capacity of flagellate protists, may lead to innovative pathways for other research areas with potential spin-offs for industrial and commercial use. Key words: flagellate protists, hindgut symbiotic fauna, lignocellulose digestion, subterranean termites Sónia Duarte1,2 (email:[email protected]), Lina Nunes1,2, Paulo A.V. Borges2, Carl G. Fossdal3 & Tânia Nobre4, 1LNEC, National Laboratory for Civil Engineering, Structures Department, Av. do Brasil, 101, PT-1700-066, Lisbon, Portugal, 2cE3c, Centre for Ecology, Evolution and Environmental Changes / Azorean Biodiversity Group and University of the Azores, Faculty of Agrarian and Environmental Sciences, PT- 9700–042 Angra do Heroísmo, 3NIBIO Norwegian Institute for Bioeconomy Research, Division for Biotechnology and Plant Health, Forest Health, NO-1431 Ås, Norway, ICAAM – Institute of Mediterranean Agricultural and Environmental Sciences, Laboratory of Entomology, University of Évora, Évora, Portugal. INTRODUCTION areas of high diversity are located in the tropics, particularly in Africa, South America and Asia Termites are social insects closely related to (Krishna et al. 2013). Termites can be informally cockroaches, from which they evolved (Lo et al. divided into two groups: lower (all families but 2000; Inward et al. 2007a; Engel et al. 2009) and Termitidae) and higher termites (Termitidae), are denominated as Polyneoptera a monophyletic based on the presence or absence of flagellate group including the cockroaches, Dermaptera, protists in their hindgut, respectively, and also on Plecoptera, Orthoptera, Embioptera,Phasmatodea, different feeding and nesting habits, and different Mantophasmatodea, Grylloblattodea, Mantodea, intestinal compartmentalisation (Eggleton 2011; and Zoraptera. More than 3,000 species of Hongoh 2011; Krishna et al. 2013). These insects termites have been described globally, but their are abundant in many terrestrial ecosystems, 21 Duarte et al. particularly in the tropics where they are a retained their bacterial symbionts, but lack the dominant invertebrate group that heavily protozoan gut symbionts. They have various contributes to the lignocellulose decomposition feeding habits, with clear separation of feeding process – thus have been called ‘ecosystem and nesting sites, and exhibit a highly engineers’ (Eggleton 2011; Palin et al. 2011). compartmentalised intestine (except for Termites also have a major role in diverse Macrotermitinae and Sphaerotermitinae). ecosystem functions, such as nutrients and Dissimilarly, lower termites feed strictly on organic matter cycling and redistribution, soil lignocellulose and are aided by hindgut symbionts fertility promotion, generation and regulation of during the digestion process; they are considered soil biodiversity and ecosystem restoration to be an ancestral branch of termites which comprises 11 families (Krishna et al. 2013). (Zimmermann et al. 1982; Bignell & Eggleton Lower termites have a dilated section of the 2000; Sugimoto et al. 2000; Jouquet et al. 2011). anterior hindgut (the paunch) where the bulk of They are major contributors to the ecologic symbiotic microbiota is harboured. Most lower stability of their habitats. By preparing different termites nest and feed within the same wood substrates, like wood or leaf-litter, into forms resource. With potential impact on within-nest easily accessed by microorganisms, termites play endosymbiont transmission, lower termites rely a major role as ecosystem conditioners (Lawton on regurgitation of crop contents and saliva et al. 1996). Termites are able to degrade (stomodeal trophallaxis) as well as proctodeal lignocellulose efficiently (e.g. Ohkuma 2008; trophallaxis, involving anus-to-mouth exchanges Husseneder 2010; Watanabe & Tokuda 2010) and of hindgut fluids, to pass food and gut contents to their feeding habits span a gradient from sound nest mates, whereas higher termites rely mainly wood to other lignocellulosic plant materials with on stomodeal trophallaxis (Eggleton 2011; different humification gradients, such as plant Shimada et al. 2013; Mirabito & Rosengaus litter or soil (Sleaford et al. 1996). This niche 2016). Proctodeal trophallaxis fosters the social, differentiation has allowed termites to promote an nutritional and symbiotic fauna interactions impact on the global terrestrial carbon cycle, among lower termites belonging to the same exceeding the cumulative decomposition roles of colony, probably playing a key role in the other arthropods (Bignell et al. 1997). integration of the information of these different Efficient lignocellulose utilisation as a food environments (Nalepa 2015). Trophallaxis may source by termites is possible because of the be horizontal, among nestmates, or vertical, establishment of endo- and ectosymbiosis, among parents and offspring. including microorganisms of all major taxa. These symbionts are Archaea and Bacteria, and LOWER TERMITES GLOBAL IMPACT protists: unicellular eukaryotes belonging to two Because of their feeding habits and preferences, separate lineages, the parabasalids and the lower termites have an important ecological oxymonads (Fig.1) (Bignell 2000; Bignell & impact on diverse ecosystems, but are also Eggleton 2000; Brune & Ohkuma 2011; Adl et al. considered to be structural, agricultural and 2012; Brune 2013). Ectosymbiosis has evolved in forestry pests, as they attack cultivated plants and the fungus-growing termites (Macrotermitinae) forest nurseries (Rouland-Lefèvre 2011). Lower which cultivate a basidiomycete fungus termites account for 80% of the economically (Termitomyces spp.) (e.g. Nobre & Aanen 2012), important species known to cause major problems whereas the majority of the other termites rely in artificial constructions (Nobre & Nunes 2007; solely on endosymbiosis. The fauna harboured Rust & Su 2012). There is concern that the inside the hindgut assists the termite host with number of invasive termite species has increased energy metabolism, nitrogen and vitamin supply more than 50% since 1969 (Evans et al. 2013), and also additional defence mechanisms (Salem which may be related to the globalisation of trade. et al. 2014, Peterson et al. 2015, Zheng et al. In 2010, the global economic impact of termites was 2015). estimated at 35.6 billion euros, and subterranean Higher termites, account for nearly 75% of termites accounted for 80% of this figure, i.e. Isoptera species richness and yet belong to a approximately 24 billion euros (Rust & Su 2012). single family, Termitidae. These termites have 22 Living inside termites Living Fig. 1. Possible evolutionary trajectory, from a most recent common ancestor of the cockroach type, of the simplified feeding groups of termites (LWT – Lower termites; FGT – Fungus 23 -growing termites; OHT – Other higher termites; SFT – Soil-feeding termites) based on their symbiotic fauna [based on: Donovan et al. 2001, Eggleton & Tayasu 2001, Inward et al. 2007b, Bujang et al. 2014; Otani et al. 2014]. A= acquisition of externalised gut through the establishment of a mutualistic external symbiosis with basidiomycete fungi; B=acquisition of flagellate protists; C=loss of flagellate protists; D=acquisition of strict soil-feeding habits. Some subfamilies have representatives in more than one feeding group. Duarte et al. As the human population increases, production, LIGNOCELLULOSE DIGESTION trade and use of wooden structures and bio- products susceptible to termite infestation FEEDING SUBSTRATE increases, potentially increasing the spread of Wood is a natural material composed of three termite pest species. Warmer seasons and changes main types of components: cellulose (framework in precipitation patterns due to climate change are substance), hemicellulose (matrix substance expected to influence termite territory size and present between cellulose microfibrils) and lignin distribution. For example, the known 27 species (incrusting substance for cell wall solidification). of invasive termite species are likely to increase For the digestion of these main components, their ranges (Su & Scheffrahn 2000; Evans et al. several enzymes
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